Without limiting the scope of the invention, the background is described in connection with the analysis of gases used in the manufacture of integrated circuits.
Heretofore, in this field, a variety of analytical methods are used in the trace analysis of gases used in the manufacture of integrated circuits. Principal figures of merit for these techniques include the extent to which they can analyze the wide variety of gases used in semiconductor manufacture, their breadth of sensitivity to the variety of contaminants thought detrimental to semiconductor processing, and their ability to quantitative trace levels of those contaminants. Among the most sensitive methods for quantitation of trace impurities is the atmospheric-pressure ionization mass spectrometer, often providing several orders of magnitude reduced limits of quantitation over alternative analytical methodologies.
The atmospheric-pressure ionization mass spectrometer relies upon the selective ionization of contaminants at atmospheric pressure with subsequent mass separation and detection using the mass spectrometer. The ionization process consists of the removal of a negatively-charged electron from a neutral molecule to create a positively-charged ion. Although the primary ionization step occurs without selectivity, the unusually high (atmospheric) pressure ensures a multitude of collisions between charged and uncharged particles resulting in the opportunity for secondary, and potentially much more selective, ionizations. The unselective primary ionizations are almost exclusively of the pure bulk gas. Consequently, a requirement for the selective secondary ionization of the uncharged trace contaminants is the energetic favorability of a charge (electron) transfer between an ionized bulk gas molecule and an uncharged contaminant molecule.
The ionization potential (IP) describes the energy required to remove the most weakly held electron from an uncharged molecule and, conversely, the energy released when an electron is supplied to a positively-charged ion. Therefore, a requirement for energetic feasibility of this process is that the IP of the contaminant of interest must be lower than that of the bulk gas. Under this condition, the ionization process can proceed with tremendous selectivity, often allowing unit ionization efficiencies for the relatively low-IP contaminant. Additionally, providing that the various contaminant IPs are appropriate, the broad detection capabilities of the mass spectrometer lend the technique to concurrent detection of a wide variety of contaminants.
Unfortunately, the IP requirement has significantly limited the scope of use for conventional atmospheric-pressure ionization mass spectrometers. While analysis of highly-detrimental contaminants such as oxygen (IP=12.07 eV) and moisture (IP=12.61 eV) in nitrogen (IP=15.6 eV), argon (IP=15.8 eV) and He (IP=24.6 eV, highest known) are typical, analysis of contaminants such as nitrogen and moisture in oxygen is unavailable. Similarly, this limitation has excluded the use of this technique to analyze contaminants of interest in the broad variety of typically low IP semiconductor process gases, such as ammonia (IP=10.2 eV), arsine (IP=10.3 eV) and borane (IP=11.4 eV).
A desirable improvement would be a modification which retains the exceptional breadth of sensitivity to various contaminants and limits of quantitation while allowing for analysis of a greater variety of bulk gases. This, in turn, would provide a reduced cost of ownership in the form of a reduced need for competing technologies to provide these additional analytical capabilities. However, it is the same energetic of the relative IPs which lends atmospheric-pressure ionization its inherent limits of quantitation that precludes its use in this extension to low ionization bulk gases.
Therefore, what is needed is a method for atmospheric-pressure ionization which does not rely on a desirable relationship between the ionization potential of the trace impurity and the ionization potential of the bulk gas.